Recently, there has been a focus on flat secondary batteries (below called flat batteries) which are light weight and thin. Flat batteries include battery elements, battery tabs (electrode terminals), and battery exteriors which are formed by laminated sheets. The battery's exterior is formed by jointing together the outer peripheral edges of the two laminated sheets, and enclosing the battery's elements and electrolyte. Battery elements are provided by laminated electrodes in which positive electrodes including a positive electrode active material layer and negative electrodes including a negative electrode active material layer are alternately stacked by means of spacers. The electrode tab (electrode terminal) is connected to the battery's elements and conducts current for the battery's elements from inside the battery to outside the battery.
A battery pack may be formed from a plurality of flat batteries. The flat batteries are stacked, and the companion electrode tabs are connected by welding and the like. The battery pack is appropriate for use with electric automobiles which require high power or hybrid automobiles and the like. With this type of battery pack, the voltage detection line is connected to the electrode terminal of every flat battery, and voltage is detected by a control device. Based on the detection results, charging and discharging of every flat battery is controlled.